Method and apparatus for generating power and low pressure saturated or near saturated steam

Abstract

Low pressure steam leaving a turbine (108) is used to preheat a major part of the feed water to a boiler (107) to a higher temperature than the balance of the feed water. The preheated part of the feed water is then introduced into the boiler (107) at a higher temperature zone than the remainder of the feed water.

What is claimed is: 
     
       1. A method for generating power and low pressure saturated or near saturated steam, which method comprises the steps of: (a) heating feed water in a boiler to produce superheated steam; and   (b) expanding said superheated steam through a turbine to provide mechanical and/or electrical power and low pressure steam; characterized in that said method includes the steps of:     (c) using at least part of said low pressure steam to heat a major part of said feed water to a temperature higher than the remainder of said feed water; and   (d) introducing the thus heated part of said feed water and the remainder of said feed water into said boiler at different temperature zones therein.   
     
     
       2. A method according to claim 1, wherein said major part comprises from 51% to 90% by volume of the feed water. 
     
     
       3. A method according to claim 2, wherein said major part comprises from 60% to 87% by volume of the feed water. 
     
     
       4. A method according to claim 3, wherein said major part comprises from 65% to 75% by volume of the feed water. 
     
     
       5. A method according to claim 1, wherein the heated part of the feed water from step (c) is added to the remainder of the feed water once it has been heated to substantially the same temperature as the heated part of the feed water. 
     
     
       6. A method according to claim 1, wherein the expanded steam leaving said turbine is superheated. 
     
     
       7. A method according to claim 6, wherein the major part of the feed water is heated first by condensing low pressure steam and subsequently by heat exchange with low pressure superheated steam from said turbine. 
     
     
       8. A method according to claim 7, (i) wherein the major part of the feed water is heated by condensing low pressure steam (ii) part of the heated feed water is further heated by heat exchange with low pressure superheated steam from the turbine; and (iii) the further heated part of the feed water, the portion which has only been heated by condensing low pressure steam, and the balance of the original feed water are introduced into the boiler at different temperature zones therein. 
     
     
       9. A method according to claim 7, (i) wherein the entire feed water is preheated by condensing part of the low pressure steam; (ii) the major part of the preheated stream is further heated by heat exchange with low pressure superheated steam from said turbine; and (iii) the further heated part of the feed water and the balance of the feed water are introduced into the boiler at different temperature zones therein. 
     
     
       10. An apparatus for generating power and low pressure saturated or near saturated steam which apparatus comprises: (a) a boiler for heating feed water to produce superheated steam; and   (b) a turbine through which, in use, superheated steam from said boiler can be expanded to provide mechanical and/or electrical power and low pressure steam; characterized in that said apparatus further comprises:     (c) a first heat exchanger arranged to receive, in use, low pressure steam from said turbine;   (d) means for conveying a major part of said feed water into said first heat exchanger;   (e) a line to convey heated feed water from said first heat exchanger to said boiler; and   (f) means to introduce the remainder of said feed water into said boiler; the arrangement being such that, in use, the heated feed water from the first heat exchanger enters said boiler at a higher temperature zone than the remainder of said feed water.     
     
     
       11. An apparatus as claimed in claim 10, including a second heat exchanger arranged, in use, to preheat feed water en route to said first heat exchanger, and a line to convey, in use, part of the low pressure steam from said first heat exchanger to said second heat exchanger to preheat said feed water. 
     
     
       12. An apparatus as claimed in claim 10, including a line to carry a first minor, portion of said feed water to said boiler, a second heat exchanger, a line to convey the balance of said feed water to said second heat exchanger, a line to convey part of said feed water from said second heat exchanger to said first heat exchanger, a line to convey hot feed water from said second heat exchanger to said boiler, and a line to convey the balance of the feed water leaving said second heat exchanger to said boiler. 
     
     
       13. An apparatus as claimed in claim 10, including a second heat exchanger, a line to convey the entire feed water to said second heat exchanger, a line to convey the major part of the feed water from said second heat exchanger to said first heat exchanger, a line to convey hot water from said first heat exchanger to said boiler, and a line for conveying the balance of said feed water leaving said second heat exchanger to said boiler.